1. Field of the Invention
The present application relates to a system and method for the active control of steady and unsteady boundary layer flow separation over surfaces of a structure.
2. Description of the Related Art
Boundary layer flows, such as flows over aircraft wings, are characterized by a thin layer of fluid adjacent a surface of a structure (i.e., the boundary layer) where viscosity of the fluid plays an important role. Outside the boundary layer viscous effects are negligible. The profile of the time-averaged streamwise velocity u(y) in the boundary layer (y being the normal distance from the surface) determines the characteristics of the boundary layer. The velocity at the outer edge of the boundary layer (y=.delta., where .delta. is the boundary layer thickness) is U (where U is the freestream velocity) and the velocity at the wall is zero (y=0). All boundary layers begin as laminar boundary layers. A laminar boundary layer is characterized by a velocity profile which has a moderate slope du/dy at the surface. After running along the surface a certain distance, a laminar boundary layer usually goes through transition to emerge as a turbulent boundary layer. A turbulent boundary layer has a much larger slope du/dy at the surface. Compared to a laminar boundary layer, a turbulent boundary layer exerts a much higher skin friction drag (.alpha.du/dy at y=0) on the surface. Delaying transition can therefore reduce skin friction.
A boundary layer separates when it encounters an opposing pressure gradient. At the separation point for steady separation, or immediately upstream of the separation point for unsteady separation the slope du/dy at the surface goes to zero. In order to delay separation, the velocity profile has to be fuller, with a high positive slope du/dy at the surface. Current measures used to delay separation increase skin friction. Conversely, measures taken to delay transition or reduce skin friction in turbulent boundary layers reduce the slope du/dy at the surface and promote separation.
Separation usually results in deterioration of performance of the structure in question. Two common examples where this may be illustrated include, air flow over an aircraft wing and air flow over automobiles. In the aircraft example, for an aircraft wing to generate lift the flow of air over the wing should follow the wing surface. However, when the aircraft is moving at slow speeds and at a high angle of attack, e.g., during take off or landing, the air flow over the top surface of the wing is prone to separate. This can result in a sudden loss in lift close to the ground. Wings of modern aircraft currently incorporate several devices, such as slotted flaps, slats and vortex generators to keep the boundary layer flow attached under these conditions. In the automobile example, flow separation over an automobile leads to the creation of a low pressure zone behind the automobile. This low pressure zone generates aerodynamic pressure drag which at typical highway speeds is the predominant form of resistance. Modern automobiles attempt to overcome aerodynamic drag by streamlining the design of the automobile. Streamlining reduces the severity of flow separation caused by abrupt shape changes which reduces the size of the low pressure zone aft of the flow separation point and hence reduces pressure drag.
As noted, flow separation may be a steady type of separation where the location of the separation point does not significantly move, or the flow separation may be an unsteady type of separation where the location of the separation point varies with time and flow conditions. Dynamic stall on helicopter rotor blades and rotating stall on axial compressor blades are examples of undesirable effects that are initiated by unsteady flow separation. Large oscillatory forces and moments are produced in both types of stall and can result in severe structural damage and erratic performance of the device if unchecked.
Traditional passive flow separation control devices, such as delta wing vortex generators and movable flaps, used to avoid or delay steady flow separation are effective for that purpose but are usually unsuitable or too cumbersome to detect and delay or avoid unsteady flow separation. For example, passive separation control devices typically induce drag even when no flow separation control is needed. Further, in order to delay or avoid unsteady flow separation, an array of passive separation control devices would be needed because the separation point in unsteady flow separation moves.
Attempts to delay or avoid unsteady flow separation have resulted in the development of active flow separation control devices. For example, an array of small imbedded nozzles have been used to eject fluid into the flow. However, in order to actively control the nozzles at the appropriate spatial location and at the appropriate point in time, an array of sensors has to be provided to track the moving separation point. These active flow control systems result in intricate mechanical systems which tend to be complex and difficult to employ. In addition, the power consumed by these systems often does not provide a net power gain because any power savings resulting from drag reduction is lost due to the power consumed by the active flow control system.
Other active systems have employed driven flexible walls for delaying boundary layer transition. However, such active systems are not believed to be capable of controlling flow separation. Separation control necessitates real time detection of the separation point and an avenue for introducing extremely small perturbations into the flow, and should be capable of exploiting natural flow processes to amplify and transform the disturbances to counteract separation even if the boundary layer is already turbulent.